The present invention relates to positioning systems, and more particular to highly precise servo positioning systems using voice coil motors, such as those employed in disc drives.
Rotating disc memories include one or more discs driven about a vertical spindle axis. Each disc typically has a plurality of concentric tracks on one or both surfaces from which information is read or onto which information is written by means of reading and/or writing heads, i.e., a transducer head assembly. The information includes servo fields providing a position feedback input to a servo system which positions the head over the surface of the disc, generally moving the head over the tracks on the surface of the disc. The transducer head assembly is typically connected to a resilient member, such as a gimbal spring, which in turn is connected to an end of a track accessing arm.
A pivot housing typically includes (or couples to) several track accessing arms. Each track accessing arm on the pivot housing carries a transducer head assembly on a first end. A second end of each track accessing arm is connected to a central portion which forms an axis of rotation between at least one actuator coils and the actuator assembly. The actuator coil is attached to a coil support structure which also extends from a side of the central portion positioned opposite the track accessing arms. The actuator itself is placed proximate to a magnetic pole piece, which includes permanent magnets and a block formed of materials having ferromagnetic properties. A variety of configurations have been proposed for such coils and magnets. See U.S. Pat. No. 5,557,152 (xe2x80x9c2-Pole Single or Dual Coil Moving Magnet Motor with Moving Back Ironxe2x80x9d) issued Sep. 17, 1995 to Raymond G. Gauthier; U.S. Pat. No. 5,448,437 (xe2x80x9cVoice Coil Motor for Disk Drivexe2x80x9d) issued Sep. 5, 1995 to Naotoshi Katahara; U.S. Pat. No. 4,775,908 (xe2x80x9cMoving Magnet Actuatorxe2x80x9d) issued Oct. 4, 1988 to John A. Ycas.
Applying a current to the actuator coil positions and holds the transducer head assembly over selected concentric tracks of the magnetic media disc. The coil is selectively energized by the disc drive system to move with respect to the magnetic pole piece. The movement of the actuator coil is transferred to the transducer heads via the actuator support structure.
In an ideal track seek operation, the servo control system of the disc drive applies current to the actuator coil which is positioned proximate to the magnetic pole piece. The current applied to the actuator coil induces a transient magnetic field which emanates from the coil and interacts with a permanent magnetic field of the magnetic pole piece. The interaction of the permanent and transient magnetic fields causes movement of the actuator coil proximate to the magnetic pole piece.
In practice, however, the force imparted to the actuator coil when current flows through it excites natural frequencies in the actuator coil. In particular, out-of-plane bending (or bending back and forth of the coil) results in an off-track error of the transducer heads since the force is being imparted to a resonating coil.
Most low frequency resonances, including out-of-plane bending of voice coils are problematic since lower frequency resonances have larger displacements. There is also less gain margin in classical second order servo control systems at lower frequencies. Insufficient gain margin can cause the servo system to go classically unstable which shows up as off-track error at the transducer head. Gain margin at resonant peaks limits the bandwidth of the servo system. A servo system with a higher bandwidth is desirable since it can more accurately follow externally induced disturbances. Thus, in order to decrease off-track error and/or increase gain margin, it is desirable to affix the voice coil in a structure having higher natural frequencies.
One problem that has impeded effective voice coil mounting is the extreme temperature variations voice coils suffer during operation. Under worst case conditions of long, fast seeks in quick succession, coil temperatures can increase enough to increase their resistance significantly. Seek times have typically been minimized through the application of relatively large amounts of current to the coil during the acceleration and deceleration phases a seek operation. One way of reducing seek time is to increase the relative amount of current to the electric coil. However, as the current is increased the operating temperature of the coil likewise increases, as a proportionate amount of the electrical energy is dissipated as heat energy. The amount of current that can be passed through a coil is generally a function of its electrical resistance, which is directly affected by the temperature of the coil. As the temperature of the coil increases, the resistance of the coil increases, and the magnitude of the control current is limited, thereby adversely affecting the drive seek time. Moreover, elevated coil temperatures can also adversely affect the seek time performance by generally weakening the strength of the magnetic circuit of the magnet assembly.
Additionally, elevated voice coil motor temperatures can result in the degradation of adhesive and insulative materials used in the construction of the voice coil motor. Such degradation can lead to internal contamination of the disc drive as well as to the shorting of the coil.
Efforts have been made to reduce such temperature increases by using external means to cool the voice coil motor. For example, U.S. Pat. No. 5,517,372 (xe2x80x9cRotating Disk Storage Device with Cooling Air Flow Controlxe2x80x9d) issued May 14, 1996 to Takeshi Shibuya et al., discloses a means for diverting air flowing over the discs to flow over the voice coil motor. However, such cooling efforts increase power consumption by creating increased drag upon the discs.
There is a continuing need in the industry for an improved actuator assembly with enhanced heat dissipation to facilitate cooling of the actuator coil without hindering the overall performance of the disc drive.
The present invention cools the actuator coil with a suitable heat sink structure. The heat sink thermally couples the coil to one or more of the head-carrying arms, which are positioned close enough to the disc stack that rotation of the stack cools them.
In one embodiment, a gap is formed between the coil and a thermal conduit, and the gap is made wide enough to allow a majority of the gap to be filed with a solid dielectric. By displacing air pockets that would otherwise form between the coil and the conduit, a higher thermal conduction and bonding strength is achieved.
In another embodiment, a thin protective layer is positioned on one of the bonding surfaces, and an adhesive (in liquid form) is also positioned between the bonding surfaces. The surfaces are then forced together, and the adhesive is cured. This is a cost-effective way to control the minimum gap thickness. In one embodiment, a layer about as thick as the coil""s cladding is applied as a coating on the irregular mounting surface of the coil. Then, a liquid adhesive having a very high metal content (i.e. more than 50% by volume) is applied to the opposite mounting surface.
In yet another embodiment, a heat conduit includes a body with a rigid arm and a rigid layer protruding from it. The body, the arm, and the layer are joined structurally so as to provide a recess into which the coil protrudes. The recess provides a large mounting area facing the coil, for highly conductive and rigid support of the coil.
Other features and advantages of the present invention will become apparent upon a review of the following figures and their accompanying description.